1. Field of the Invention
The present invention relates to methods and devices for the transportation of droplets or particles using an electric field gradient.
2. Background
The scaling down of chemical reactions, separations, and analysis processing using microfluidic devices may be useful in various areas of chemical engineering, pharmaceuticals, and biotechnology. Many of the microfluidic devices operate by microchannels inside plastic or glass, which can lead to surface fouling and other problems. The fluids of interest may be in direct contact with the plastic or glass. The liquid inside these channels generally flows in a continuous stream because high capillary pressures generated at any air-liquid boundaries in the microchannels may disrupt operation. Pumping and valving in such small channels may require a significant amount of energy because of high viscous dissipation. Therefore, many of these devices operate as continuous process devices.
Alternatives to continuous streams in microchannels include methods to move a liquid as a micro- or nano-droplet(s) using electric fields or gradients in interfacial tensions. The manipulation of microscopic droplets on a solid surface, however, may be technically difficult. For example, the contact angle hysteresis of the droplets can lead to strong capillary forces, which may increase losses of power and may pin the droplets onto surface contaminants and/or scratches. The open surface of the droplets, combined with the high capillary pressure in the droplets, may lead to rapid evaporation and/or surface fowling. In addition, molecular, particulate, or biological species inside the droplets can become adsorbed on the surface of the solid, which can lead to a loss of the component for which processing is desired, higher contact angle hysteresis and chip contamination that can be difficult to reverse.
The manipulation of microdroplets can also be based on the application of alternating current (“AC”) electric fields, called dielectrophoresis (“DEP”). DEP may be used for the manipulation, separation, and collection of cells, viruses, biomolecules and nanoparticles. AC voltages may be used to pull water droplets into a gap between liquid siphons, and similar techniques may be used to move water droplets on solid surfaces. T. B. Jones, M. Gunji, M. Washizu and M. J. Feldman, J. Appl. Physi. 89, 14A-F41-14A-F48 (2001) Dielectrophoretic liquid actuation and nanoddroplet formation.; T. B. Jones, Electrostat. 51, 290-299(2001) Liquid dielectrophoresis on the microscale.; M. Washizu, IEEE T. Ind Applic. 34, 732-737 (1998). Electrostatic actuation of liquid droplets for miroreactor applications.; M. G. Pollack, R. B. Fair, A. D. Shenderov, Appl. Phys. Lett. 77, 1725-1726 (2000) Electrowetting-based actuation of liquid droplets for microfluidic applications. Parallel electrodes can be used that operate directly on water droplets that are placed on a solid surface. The droplets can be drawn between the electrodes because of the high dielectric permittivity of water. Relatively high voltages and/or high frequencies may be required, which can lead to significant power dissipation, heating of the aqueous phase, and evaporation. In addition, the droplets are generally in direct contact with a solid surface, such as plastic or glass. Thus, many of the problems discussed above with respect to surface fouling, evaporation, chip contamination, etc. may also be present.